To relieve the operator from the burden of manually managing Neighbor Relations (NRs) and even to construct a self-optimization network (SON), a technique called ANR has been proposed and applied in cellular communications systems. One of the fundamental requirements for implementing ANR is to uniquely identify each cell in a cellular communications system.
Although Physical Cell Identifiers (PCIs) may be used for cell identification, the total amount of PCIs in current cellular systems (e.g., 504, in LTE systems) is too limited to achieve unique identification of each cell in the system. When different neighbor cells apply the same PCI, it is impossible to distinguish them by PCI and confusion results. To avoid such confusion, in addition to a Physical Cell Identifier, a Cell Global Identifier (CGI) is used to describe an effective NR for a neighbor cell. The total amount of CGIs is about 256 million, which is large enough for each cell to be identified by a unique CGI.
In this regard, the ANR process requires ANR measurement procedure to successfully acquire CGI. An example of the ANR process in LTE systems is provided in [1] and will be described in the following by referring to FIG. 1. As illustrated in FIG. 1, a UE 101 currently located at the border between cell A and Cell B is served by an eNodeB (eNB) 201 and is able to measure the PCI of Cell B.
At the beginning of the ANR process, the UE 101 sends the measured PCI of Cell B to the eNB 201 (step S101). Upon receiving the measured PCI, the eNB 201 instructs the UE 101 to read the enhanced CGI (ECGI) of Cell B (step S102). In accordance with the instruction, the UE 101 receives System Information Block 1 (SIB1) from Cell B and reads the ECGI of Cell B contained in the SIB1 (step S103). Upon acquiring Cell B's ECGI, the UE 101 reports it to the eNB 201(step S104). Then, using the PCI and the ECGI of Cell B, the eNB 201 creates a new NR entry for Cell B in its NR list.
At step S103 of the above process, a continuous time gap shall be implemented, during which, instead of receiving data from Cell A, the UE 101 establishes synchronization with Cell B and maintains the synchronization to measure the ECGI.
Currently, to implement such a time gap, the 3rd Partnership Project (3GPP) organization proposes two alternative approaches: Discontinuous Reception (DRX) and autonomous gap. The DRX approach is described in [1] and [2], and the description about autonomous gap can be found in [1] and [3]. As the DRX approach is preferable to the autonomous gap approach in terms of implementation simplicity and backward-compatibility with legacy UEs and infrastructures, DRX-based ANR measurement should be applied in any phase of LTE networks.
The DRX functionality is standardized in LTE systems since Release 8. Though some other auxiliary parameters are used, the DRX functionality is mainly characterized by the three parameters as below:                On-Duration Timer, specifying an on-duration (in terms of time or in number of subframes) during which a UE waking up from DRX keeps awake to continuously attempt to receive and decode Physical Downlink Control Channels (PDCCHs). If the UE successfully decodes a PDCCH before the timer expires, it stays awake and starts a DRX Inactivity Timer.        DRX Inactivity Timer, specifying a duration (in terms of time or in number of subframes) during which the UE continuously attempts to receive and decode PDCCHs. If the UE fails to successfully decode a PDCCH until the timer expires, it re-enters into DRX. The UE shall restart the DRX Inactivity Timer following a successful decoding of a PDCCH for a first user data transmission only (i.e. not for retransmissions).        DRX Cycle, specifying the periodic repetition of the on-duration.        
FIG. 2 illustrates the above three parameters. Moreover, as shown in FIG. 2, the total time duration when UE is awake to monitor PDCCH is called active-duration, and the result of DRX cycle minus active-duration is called sleep-duration. The active-duration includes not only the “on-duration” but also the time duration during which the DRX Inactivity Timer is running. Based on the above, the minimum active-duration is of length equal to on-duration, and the maximum is undefined (infinite). Correspondingly, the sleep duration ranges from DRX cycle minus on-duration to 0.
Since the DRX functionality is originally introduced for power saving at UE, fixed DRX parameters are dedicatedly set for UEs having relatively low traffic intensity in current implementation. However, in practical situation, a UE does not necessarily employ the DRX functionality only when it is of low traffic intensity. This is especially true when a UE having high traffic intensity is to perform DRX-based ANR measurement enabling its handover to an unknown neighbor cell.
If a UE having high traffic intensity follows the fixed DRX parameters dedicatedly set for UEs having low traffic intensity, the active duration of the UE will increase while the sleep duration left for the UE will be shorten. This adversely reduces the possibility for the UE moving towards an unknown neighbor cell to successfully perform the DRX-based ANR measurement procedure, which in turn impedes the UE's handover to the unknown neighbor cell and may cause undesirable connection dropping and service interruption.